The present invention relates to a high chrome work roll manufactured by centrifugal casting, and particularly pertains to a high chrome work roll having high resistances, to wear and surface roughening and fail-safe properties as finishing work rolls including those for hot strip mills, cold strip mills and for hot skin pass, and a manufacturing method of the said roll.
As is heretofore known, for finishing work rolls employed for hot or cold rolling, etc., following characteristics are required:
(1) Wear resistance:
This is an important property having bearing on the surface property and the sheet thickness accuracy of the rolled product.
(2) Fail-safe property:
The work roll needs to have adequate toughness to abnormal rolling.
(3) Resistance to surface roughening:
Since the property of the roll surface has a large bearing on the quality of the rolled product, resistance to surface roughening is necessary.
As materials having the above listed three characteristics, Adamite, indefinitely chilled material, ductile material, chilled material, cast steel or forged and quenched steel, etc., are taken seriously and are finding wide uses, but having respective merits and demerits, they still can not be said adequate as finishing work rolls of strip mills, etc.
Thus Adamite and indefinitely chilled material, for example, in which free cementite is crystallized out in large quantities, are problematical in their resistance to the surface roughening and in their toughness. In steel in which graphite is crystallized out, such graphite falls off, resulting in surface roughening. As a remedy, uniform dispersion of hard carbides into the material is effective. For this purpose, it is advisable to have the roll made of a material in which the resistance to surface roughening and wear resistance are improved by increasing the Cr content. However, to achieve high hardness with the same material is to have high internal residual stress, causing difficulty in manufacture. So, as is well known, a composite roll formed of different materials for its barrel part and for its journal part (internal layer) is employed. Such a composite roll is generally formed by way of centrifugal casting.
When in the above-mentioned composite roll, a material having a large Cr content is used for its shell, and a steel material is used for the core, the toughness of the material will not be substantially deteriorated, even if a core with a high Cr content results from the diffusion of Cr from the inside surface of the shell into the core. However, in manufacturing the core of steel, the feeder for preventing shrinkage holes in the cope part needs to be increased, resulting in high cost, and because of high modulus of elasticity, such a material is disadvantageous in regard to the thermal stress and residual stress.
Accordingly, in order to reduce feeder, and decrease the modulus of elasticity of the core, thereby relieving the thermal stress and the residual stress, the core should desirably be formed of a cast iron, but if Cr in the shell is remelted, and diffused into the core, the core material becomes very brittle, causing the significance of the compounding to be lost. Moreover, when a high Cr material is used for the shell, an oxide film with a high melting point is formed on the inside surface of the shell, giving rise to inadequate weldability.